1. Field of the Invention
The present invention relates generally to voltage generation circuits used in integrated circuits, and, in particular, to charge pump voltage generation circuits.
2. Description of the Related Art
In general, in the descriptions that follow, I will italicize the first occurrence of each special term of art that should be familiar to those skilled in the art of integrated circuits (“ICs”) and systems. In addition, when I first introduce a term that I believe to be new or that I will use in a context that I believe to be new, I will bold the term and provide the definition that I intend to apply to that term. In addition, throughout this description, I will sometimes use the terms assert and negate when referring to the rendering of a signal, signal flag, status bit, or similar apparatus into its logically true or logically false state, respectively, and the term toggle to indicate the logical inversion of a signal from one logical state to the other. Alternatively, I may refer to the mutually exclusive boolean states as logic_0 and logic_1. Of course, as is well known, consistent system operation can be obtained by reversing the logic sense of all such signals, such that signals described herein as logically true become logically false and vice versa. Furthermore, it is of no relevance in such systems which specific voltage levels are selected to represent each of the logic states.
In general, a charge pump performs power conversion. In particular, an AC-to-DC charge pump draws power from an alternating current (“AC”) source to develop one or more direct current (“DC”) power supplies for load circuitry. Typically, the regulation for these power supplies is not within the charge pump proper. Rather, regulation is often provided by a regulator which will spill excess current so as to maintain a steady DC supply voltage.
Shown in FIG. 1 is a typical integrated system 10 comprising antenna 12, tank circuit 14, AC-to-DC charge pump 16, DC regulator 18, and an exemplary load circuit 20. As is known, to be most efficient, the resonant frequency of the tank circuit 14 must be tuned to the carrier frequency of a received radio frequency (“RF”) signal. One effective technique for dynamically tuning the resonant frequency is disclosed in the Related Co-Application.
During operation, as the charge pump 16 draws more current from the tank circuit 14, the Q will drop, and the available antenna voltage will decrease. Ultimately, the current which can be supplied by the charge pump 16 is limited by the RF power received by the antenna 12.
The primary requirement of charge pump 16 is to achieve the targeted supply voltage from the smallest possible antenna signal (high gain) at the highest possible efficiency. High gain is not required simply because the antenna signal is small. Unloaded, the very high Q of the tank circuit 14 can easily achieve voltages in excess of several volts. As the charge pump 16 is energized, the load 20 will pull current from the antenna 12 through the charge pump 16. The effective input impedance of the charge pump 16 will decrease, causing the Q of the system 10 to drop until the input voltage stabilizes at the input voltage that will just support the current drawn by the regulator 18 and the load 20. The higher the gain of the charge pump 16, the smaller the input signal required to sustain the circuit load.
Shown in FIG. 2 is a prior art 2-diode (i.e., second-order) charge pump 16a, comprising capacitors 22 and 24, and diodes 26 and 28. The idealized output voltage of charge pump 16a is:Vo=2*(Vp−Vd)  [Eq. 1]where Vp is the (peak-differential) input voltage and Vd is the forward diode drop of diodes 26 and 28. The derivation is left to the reader. A somewhat more accurate expression for the output voltage is:Vo=2*(a*Vp−Vd)  [Eq. 2]where a is the AC-coupling gain (hereinafter referred to as “coupling efficiency”) from the VINP input to the node 30 (hereinafter referred to as a “flying node”). For convenience of reference, we shall hereinafter refer to any node having a voltage that is substantially static with respect to VSS as a “fixed bias node”. Since VSS is static with respect to itself, VSS is, by this definition, a fixed bias node.
The input capacitor 22 should clearly be chosen to make a very close to 1. We can see that:
                                          V            p                    ⁢          min                =                              1            a                    *                      (                                                            V                  o                                2                            +                              V                d                                      )                                              [                  Eq          .                                          ⁢          3                ]            
Even with Vd˜0, the 2-diode charge pump 16a would require an input of at least
      (                  1        a            *                        V          o                2              )    ,or about 1 Vp to sustain a 1.8 V output.
To a good approximation, on each cycle all of the load current (for that cycle) is drawn through both diodes, and the load current is pulled from Vin twice (once each half cycle) while Vin is at its maximum (Vp). Therefore, neglecting a, the total power drawn from the input is:Pin=2*Vp*Iload  [Eq. 4]Also, the total power lost in charge pump 16a (the power dissipated in the diodes during forward conduction) is approximately:Pd=2*Vd*Iload  [Eq. 5]Finally, the power delivered to the load is:Pi=Vo*Iload  [Eq. 6]and the power efficiency is:
                    PE        =                                            P              l                                      P                              i                ⁢                                                                  ⁢                n                                              =                                    1              -                                                P                  d                                                  P                                      i                    ⁢                                                                                  ⁢                    n                                                                        =                                                            V                  o                                                  (                                      2                    *                                          V                      p                                                        )                                            =                                                V                  o                                                  (                                                            V                      o                                        +                                          (                                              2                        *                                                  V                          d                                                                    )                                                        )                                                                                        [                  Eq          .                                          ⁢          7                ]            Clearly, to maximize efficiency you must minimize the forward drops of diodes 26 and 28.
Shown in FIG. 3 is a prior art 3-diode (i.e., third-order) charge pump 16b, comprising capacitors 32, 34 and 36 and diodes 38, 40 and 42, with a flying node 44. The analysis of this circuit is a bit more complex, but it can be shown that:
                              V          o                =                  3          *                      (                                          V                p                            -                              V                d                                      )                                              [                  Eq          .                                          ⁢          8                ]                                                      V            p                    ⁢          min                =                              (                                          V                o                            3                        )                    +                      V            d                                              [                  Eq          .                                          ⁢          9                ]                                PE        =                                            V              o                                      (                              3                *                                  V                  p                                            )                                =                                    V              o                                      (                                                V                  o                                +                                  (                                      3                    *                                          V                      d                                                        )                                            )                                                          [                  Eq          .                                          ⁢          10                ]            As can be seen, for higher-order charge pumps, the minimum input voltage is reduced at the cost of decreasing efficiency.
Both the 2-diode and the 3-diode charge pumps suffer from a practical problem, which is that with VINN tied to VSS, VINP swings Vp above and below VSS. If VSS is tied to the substrate, VINP will tend to forward bias all substrate diodes on VINP on the down swing, unless the Vp required to reach the target output voltage is very small indeed.
I submit that what is needed is an improved charge pump that provides improved power efficiency while overcoming the problems discussed above.